Radar-equipped field guns and Katyusha rocket launchers in Syrian- and Muslim-controlled districts are activated the minute freighters approaching Christian-held seaports are detected on the screens. Christian-led military units respond by pounding Muslim areas with heavy artillery.

The fire is often also directed at Syrian strongholds in the Bekaa region in eastern Lebanon, where half of Syria's 40,000-strong task force in Lebanon is deployed. The rest are stationed in mainly Muslim West Beirut and the north of the country.

Voice of Lebanon, the main Christian radio station, said Syrian gunners Monday night intensified their shelling of the Christian enclave after Syrian ground-to-air missiles in the Bekaa received direct hits. The report could not be verified independently, and Syrian officers here were not available for comment. Death Toll Reaches 335

For four days, artillery shells and rockets rained on most of Lebanon, including residential areas on both sides of the Green Line, that splits Beirut into Muslim and Christian halves.

The police estimated that 2,200 shells and rockets were fired by the combatants in the last 72 hours. Syrian and Muslim gunners today resumed their bombardment of the Beirut port and two other harbors north of here, Junieh and Byblos, with Christian forces responding in kind.

According to police records, 25 people have been killed and about 125 wounded in the last three days, raising the toll in the most recent eight weeks of violence here to 335 dead and 1,100 wounded.

The violence dashed hopes in a cease-fire called 11 days ago by foreign ministers of the 25-nation Arab League. Lebanese are still waiting for arab observers to come here to monitor and stabilize the truce. Deployment of Observers

The secretary general of the Arab League, Chedli Klibi of Tunisia, has served notice that he will not send the observers until the warring factions in Lebanon stop shooting. The observers will be from Tunisia, Algeria, Jordan, the Sudan and and Yemen.

Mr. Klibi is sending a special representative, Lakhdar Ibrahimi, to Beirut and the Syrian capital, Damascus, to discuss deployment of the observers on Lebanese confrontation lines.

Senior Lebanese Muslim officials were in Damascus today for talks with leaders there. Selim al-Hoss, prime minister of a mainly Muslim cabinet, and Hussein al-Husseini, the Speaker of Parliament, met with the Syrian Vice President, Abdel Halim Khaddam, the Syrian state radio reported. Nabih Berri, leader of the mainline Shiite militia Amal, and Walid Jumblat, head of the predominantly Druse Progressive Socialist Party, are also in the Syrian capital.